Thermally assisted magnetic recording (TAMR) is expected to be capable of recording at 1˜10 Tb/inch2 data density. TAMR converts optical power into localized heating of the magnetic recording medium to temporarily reduce the field needed to switch the magnetizations of the grains that comprise the medium. Through a steep temperature gradient alone, or in conjunction with a high magnetic field gradient, data storage density can be improved to well beyond the current state-of-the-art magnetic recording technology.
A TAMR head, in addition to the conventional magnetic recording features, usually comprises an optical wave-guide (WG) and a Plasmon antenna (PA) or Plasmon generator (PG). The WG acts as an intermediate path to guide an external laser light to the PA or PG, where the light optical mode couples to the local Plasmon mode of the PA or to the propagating Plasmon mode of the PG. The optical energy, after being transduced to Plasmon energy, either with local Plasmon excitation in the PA or with energy transmission along the PG, is then concentrated at the location where medium heating happens. TAMR can be achieved when the heated spot is correctly aligned relative to the magnetic field from the magnetic recording structures.
Prior art [1-2] proposals for a head structure to realize TAMR are shown in cross-section in FIG. 1a, with an ABS view in FIG. 1c while FIG. 1b shows the laser diode as seen when looking up from the slider. Seen in these figures is magnetic read head 11, PA (or PG) 12, optical wave-guide (WG) 13, write pole 14 and laser diode (LD) 15 that is mounted on the top of the slider by a fixture. The beam emitted by laser diode 15 emerges from internal wave-guide 16, is polarized along direction 17 and couples into WG mode in the slider. The PG needs to be initiated by the correct edge plasmon (EP) mode i.e. one in which the major electric field component 17 is along the y-direction, as shown in FIG. 1a, in order to be able to confine the optical energy at the end of the sharp tip. For this purpose, both the WG and LD optical modes should also have the correct polarization, which is again along the y-direction.
In other words, LD 15 needs to be operated in the transverse magnetic (TM) mode. LDs are not usually designed and fabricated for operation in the TM mode and their cost is often higher than that of LDs operating in the more commonly used transverse electric (TE) mode (whose polarization is in the x-direction). Since it is normally preferred to operate in the cheaper LD mode, the invention discloses a novel and efficient plasmon generator suitable for operating in conjunction with the more common TE mode LD.
Other critical limitations to the achievement of high optical efficiency have been documented in the prior art. First, the efficiency of coupling WG light into the edge plasmon mode is limited by mode size mismatch between the diffraction-limited optical mode in the WG and the sub-diffraction-limited edge plasmon mode in the PG. Because the WG mode is much larger than the highly confined edge plasmon mode, only a small fraction of the optical energy can be transferred.
Second, propagation loss of the edge plasmon mode along the PG can be significant due to the high confinement of the edge plasmon mode. Third, the coupling and propagation efficiencies of the edge plasmon mode tend to be very sensitive to edge variations in PG 12, so tight fabrication tolerance is required. Fourth, the plasmon antenna or the PG tend to be isolated and of limited volume so thermal management of these components to avoid heat damaging them could be challenging. The present invention discloses how the overall optical efficiency of the plasmon device can be improved so that less laser power will be consumed in the TAMR head.
In a recent paper by Ginzburg et al. [3], a three-section structure is described. However, radiation propagating through it is not compressed in a direction normal to the plane in which these three sections lie, so only (conventional) 2-D focusing is provided. Also, the authors use the terms ‘TM’ and ‘TE’ (to define modes) differently from their usage in the present invention. In our case, for example, the TM mode has its dominant electric field component (i.e. its plane of polarization in the optical sense) perpendicular to the diode film stack, i.e. parallel to the film's original growth direction (see direction 17 in FIGS. 1a and 1b) with the in-plane of polarization for the TE mode being parallel to said stack. The diode stack's original growth direction is relevant because it determines the distribution of internal stress in the laser diode.    [1] K. Tanaka, K. Shimazawa, and T. Domon, “Thermally assisted magnetic head,” US Patent Pub. #2008/0192376 A1 (2008)    [2] K. Shimazawa, and K. Tanaka, “Near-field light generator plate, thermally assisted magnetic head, head gimbal assembly, and hard disk drive,” US Patent Pub. #2008/0198496 A1 (2008)    [3] P. Ginzburg, D. Arbel, and M. Orenstein, “Gap plasmon polariton structure for very efficient microscale-to-nanoscale interfacing,” Opt. Lett. 31, 3288-3290 (2006)□
A routine search of the prior art was performed with the following references of interest being found:
In US 2009/0116804, Peng et al. disclose a plasmon generator having TE mode while Kim et al. describe a plasmon generator having a gap in US 2005/0062973. U.S. Pat. No. 7,042,810 (Akiyama et al.) teaches a laser oscillated in TE mode in TAMR while US 2008/0192376 (Tanaka et al.) and US 2008/0198496 (Shimazawa et al) disclose plasmon antennas having a triangle shape.